Friday, October 20, 2017

PLoS Comp. Bio.: Spring & Early Summer Most Likely Time For A Pandemic

Credit Spencer J. Fox

#12,842

We've a new study, just published in PLoS Computational Biology, that looks at the history of 6 pandemics in the Northern Hemisphere since 1889, and finds they all first emerged in spring and early summer. Using a computer model, the authors found evidence of a narrow window of opportunity for pandemic emergence.

The authors then proposed two possible factors behind this trend, one of which long time readers of this blog will recall was a frequent topic of conversation after the last pandemic.

First some excerpts from a press release from the University of Texas At Austin, and a link and some excerpts from the study, then I'll return with a jaunt down memory lane.

You might expect that the risk of a new flu pandemic — or worldwide disease outbreak — is greatest at the peak of the flu season in winter, when viruses are most abundant and most likely to spread. Instead, all six flu pandemics that have occurred since 1889 emerged in spring and summer months. And that got some University of Texas at Austin scientists wondering, why is that?

Based on their computational model that mimics viral spread during flu season, graduate student Spencer Fox and his colleagues found strong evidence that the late timing of flu pandemics is caused by two opposing factors: Flu spreads best under winter environmental and social conditions. However, people who are infected by one flu virus can develop temporary immune protection against other flu viruses, slowing potential pandemics. Together, this leaves a narrow window toward the end of the flu season for new pandemics to emerge.

The researchers’ model assumes that people infected with seasonal flu gain long-term immunity to seasonal flu and short-term immunity to emerging pandemic viruses. The model incorporates data on flu transmission from the 2008-2009 flu season and correctly predicted the timing of the 2009 H1N1 pandemic.

Influenza pandemics can emerge unexpectedly and wreak global devastation. However, each of the six pandemics since 1889 emerged in the Northern Hemisphere just after the flu season, suggesting that pandemic timing may be predictable.

Using a stochastic model fit to seasonal flu surveillance data from the United States, we find that seasonal flu leaves a transient wake of heterosubtypic immunity that impedes the emergence of novel flu viruses. This refractory period provides a simple explanation for not only the spring-summer timing of historical pandemics, but also early increases in pandemic severity and multiple waves of transmission.

Thus, pandemic risk may be seasonal and predictable, with the accuracy of pre-pandemic and real-time risk assessments hinging on reliable seasonal influenza surveillance and precise estimates of the breadth and duration of heterosubtypic immunity.

Eight years ago, months after the 2009 H1N1 pandemic had emerged - but a couple of months before the monovalent H1N1 vaccine would be available - news of an unpublished Canadian study began to surface that suggested that those who had received a seasonal flu shot the previous year were more likely to contract the new pandemic virus than those who hadn’t.

TheCDC and the World Health Organizationboth looked at their data, and issued statements that they could find no correlation between the seasonal vaccination and increased susceptibility to the pandemic flu.

With concerns rising, a number of Canadian Provinces halted or announced delays in rolling out the seasonal flu shot, even though the study had yet to be published (see Ontario Adjusts Vaccination Plan).

The debate raged on, with conflicting data (seehere, here, and here), long after the 2009 pandemic ended.

In November of 2010, an article appeared in the Eurosurveillance Journal (seeEurosurveillance: The Temporary Immunity Hypothesis) that suggested that contracting seasonal flu (as opposed to being vaccinated against it) temporarily ramped up the body’s immune system against other viruses – and that this protective effect could last months.

Unlike the Canadian researchers, these scientists could find no increased susceptibility to the pandemic H1N1 virus among Australians who had been vaccinated the previous year against seasonal flu. The difference between the two findings, they posited, came from three separate factors:

A theory regarding temporary immunity following any influenza infection

The timing of the arrival of the pandemic virus in Canada

And the protective effects of seasonal flu vaccination against seasonal - but not pandemic - flu.

While unproven, this hypothesis fits in nicely with the findings of today's study.

While there could be other factors we don't know about that might override this proposed narrow window of opportunity for pandemics - based on the historical record and the growing evidence for the temporary immunity hypothesis - late spring and early summer do seem the most likely time for pandemic emergence.